Strength of Pulmonary Capillaries

There is now good evidence that the strength of the thin side of the blood-gas barrier comes not from the cellular layers themselves, but from the interstitium or extracellular matrix, and in particular the type IV collagen in the basement membranes. Here it is important to appreciate that in the thin side of the blood-gas barrier, the basement membranes of the alveolar epithelium and capillary endothelium fuse in the center of the extracellular matrix to give a central electron-dense band rich in type IV collagen perhaps only 50 nm thick. It is believed that this extremely thin sheet of type IV collagen is mainly responsible for the mechanical strength of the thin side of the blood-gas barrier.

The evidence for this can be summarized as follows. We have found that in animal studies where the capillary pressure is raised to high levels, disruptions can be seen in the epithelial and endothelial layers but often the basement membrane remains intact [4, 5]. Additional evidence comes from studies on isolated rabbit renal tubules that show that the distensibility of the tubules is determined by their basement membrane [6]. In addition, studies of the distensibility of frog mesentery capillaries show that this is attributable to the mechanical properties of the basement membrane. Further studies show that the thickness of the basement membrane of systemic capillaries increases down the body as the

Epithelium

Figure 2 Diagram showing the two mechanisms that can increase the mechanical stress in the capillary wall. © shows the hoop or circumferential stress as a result of an increase in the capillary transmural pressure. @ indicates the longitudinal tension in the alveolar wall as the lung is inflated to a high volume. Part of this tension is transmitted to the capillary wall. (From Ref. [5].)

Epithelium

Figure 2 Diagram showing the two mechanisms that can increase the mechanical stress in the capillary wall. © shows the hoop or circumferential stress as a result of an increase in the capillary transmural pressure. @ indicates the longitudinal tension in the alveolar wall as the lung is inflated to a high volume. Part of this tension is transmitted to the capillary wall. (From Ref. [5].)

pressure within them rises. Finally, it is well known that renal glomerular capillaries have a thick basement membrane consistent with their high transmural pressure.

What is known about the structure and ultimate tensile strength of type IV collagen? It has a triple helix structure similar to that of other matrix collagens, but is unusual in that two molecules join at the C-terminal end to give a doublet about 800 nm long, and four molecules join at the N terminal to give a matrix configuration rather like chicken wire. There are few studies of the ultimate tensile strength of basement membrane, but measurements on cat lens capsule together with studies on isolated rabbit renal tubules suggest that the ultimate tensile strength is approximately 1 x 106Nm-2, a very large value. Thus type IV collagen is ideally suited to confer the high tensile strengths needed in the wall of the summary capillary.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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